1. Field of the Invention
This invention relates to a temperature control system for fixing-drying device or the like using heat in copiers, printers or other machine.
For the purpose of illustration, the invention will hereinafter be described with respect to an electrophotographic copying machine, but it should be understood that the invention is not limited thereto.
2. Description of the Prior Art
In an electrophotographic copying machine, a drum having a photosensitive layer in uniformly charged with corona discharge, whereafter the drum is exposed to image light to form an electrostatic latent image thereon. Toner is deposited on such electrostatic latent image to develop the same into a visible image, which is then transferred to ordinary paper. Such paper with the image transferred thereto is heated by a heat source to fix the toner on the paper. These are the typical steps of the process.
There are various types of the fixing device, such as type of using infrared ray lamps, type of using heated wind blower, type of employing metal plates of aluminum, copper, iron or the like heated by nichrome wires.
FIG. 1 of the accompanying drawings shows one of such conventional heating devices. The fixing device 11 comprises a plurality of infrared ray lamps 12, which are grouped into upper lamps 14 located above a sheet of paper 13 within the fixing device and lower lamps 15 located below the paper 13. The lower lamps 15 are normally turned on and the upper lamps 14 are turned on and off under the control of an output corresponding to a temperature as detected by a thermosensitive element 16 located within the fixing device. Thus, the temperature within the fixing device 11 is such as shown in FIG. 2, wherein it is seen: the upper and lower lamps are all turned on until a set temperature TO is reached; the upper lamps are turned off at a point of time ta whereat the temperature within the device 11 reaches TO, but the temperature is somewhat raised by remaining heat and then falls; the upper lamps are again turned on after a point of time tb is passed and the interior temperature becomes lower than the set level, whereas the temperature still continues to fall for some time because the upper lamps cannot immediately raise the temperature; after a predetermined length of time has passed, the temperature begins to rise and after a point of time tc is passed, the upper lamps are again turned off. By repetition of such operation, the temperature within the fixing device is maintained substantially at TO. According to such system, however, the temperature distribution is irregular in such a manner that the temperature in the area above the paper located within the device is higher when the upper lamps are in ON condition, but lower when these lamps are OFF condition.
FIG. 3 shows another example of the conventional heating device, in which first nichrome wires 31 normally with a current applied thereto and second nichrome wores 33 ON-OFF controlled by the output from a thermosensitive element 32 are alternately arranged, an insulative material 34 surrounds these nichrome wires 31 and 33 and a metal plate 35 extends over the insulative material, whereby the metal plate 35 may be used as a radiator plate.
In such heating device, the second nichrome wires 33 are ON-OFF-controlled as in FIG. 1, and the temperature distribution within the fixing device is irregular in such a manner that the surface of the metal plate maintains a substantially uniform temperature distribution when both the first and the second nichrome wires are being heated but that when only the first nichrome wires are being energized, the portion of the metal plate adjacent the first nichrome wires is at a higher temperature while the portion of the metal plate adjacent the second nichrome wires is at a lower temperature. Further, the heat sink such as the normally heated first nichrome wires are serviced in the vicinity of a rated value of power and this reduces the durability of the heat sink.
As the means for eliminating these disadvantages, it would occur to propose to use a heat sink such as a single heater or the like and to cause the output of a thermosensitive element to continuously control the phase angle of an AC signal which is a driving signal for said heater. However, if such continuous control of the phase angle were resorted to, the driving signal would be switched at a high level point of the AC signal and this would result in production of noise. Further, according to such method, the required temperature could not quickly be obtained at the initial heating stage of the heat sink, that is, the rising would be slow.